1. Field of the Invention
This invention relates to a hermetic motor-driven compressor used in a refrigerator, an air condenser, or the like.
2. Description of the Prior Art
Generally, a hermetic motor-driven compressor comprises, as shown in FIG. 1 of the accompanying drawings, a shell or casing 1; a crankshaft 2 having eccentric portion 2a provided at its upper end and an oil passage 10 longitudinally formed therein, and rotatably driven by an electric motor (not shown) disposed within the casing 1; a reciprocating piston 3 coupled with the eccentric portion 2a of the crankshaft 2 by means of a slide member 6 which is attached to one end of the piston and slidably engaged with the eccentric portion of the crankshaft; a cylinder 4 providing a space for compressing refrigerant gas and receiving therein the piston 3 such that the piston is reciprocably movable in the cylinder with rotation of the crankshaft; and a head portion 5 connected to an opposite end of the cylinder from the piston.
In addition, the bottom of the casing 1 constitutes a sump filled with refrigeration oil in which the lower end of the crankshaft 2 is immersed. In operation of the hermetic motor-driven compressor thus constructed, as the crankshaft 2 is rotated by the motor, the piston 3 coupled with the crankshaft reciprocates within the cylinder 4, resulting in a change of pressure in the cylinder. As a result, the refrigerant gas in the casing 1 is drawn in and compressed within the cylinder, and then discharged out of the cylinder.
Also, during the operation of the compressor, the refrigeration oil in the lower portion of the casing is drawn up through the oil passage 10 formed in the crankshaft 2, while lubricating the crankshaft, and then spouted out through an opening of the upper end of the crankshaft. The spouted oil then falls downwardly to lubricate the contact surface of the piston 3 with the cylinder 4, and thereafter flows down to the sump in the lower portion of the casing.
Since during operation of the compressor the motor is driven at a high rotational speed in the range of 3200-3300 rpm, frictional heat is generated between the cylinder and the piston due to such a high speed rotation, and thus tends to elevate the temperature of the sucked refrigerant gas in the cylinder, thereby lowering volumetric efficiency and hence performance of the compressor.
In the past, attempts have been made to restrain generation of the frictional heat by supplying sufficient refrigeration oil to the contact surface of the piston with the cylinder so as to increase the lubrication effect. One such attempt is disclosed in Japanese Laid-Open Utility Model Publication No. SHO 60-139091, wherein a shell is formed on its upper surface with a protrusion of a circular arc shape centered on a central portion of the upper surface above an axis of a cylinder and protruding to a point closely adjacent to the periphery of the cylinder, whereby the refrigeration oil dashed against the protrusion is directed to a place requiring lubrication.
In another prior art as disclosed in Japanese Laid-Open Patent Publication No. SHO 56-156478, a slide tube integral with a piston is provided on its upper portion with a burring structure extending some distance above the peripheral surface of the piston such that the spouted refrigeration oil is guided by the structure. Still another prior art is shown in Japanese Laid-Open Patent Publication No. SHO 63-85270, wherein an eccentric shaft of a crankshaft and a larger diameter portion of a connecting rod engaged with the eccentric shaft both have oil passages extending transversely in a common horizontal plane.
The prior hermetic motor-driven compressors as discussed above are disadvantageous in that, while the arrangements provide somewhat effective lubrication of the contact surface of the piston with the cylinder by the refrigeration oil, they fail to satisfactorily prevent lower performance of the compressor due to frictional heat generated between the cylinder and the piston. Further, the prior arrangements additionally require a separate process of forming the protrusion on the upper surface of the steel casing, the burring structure on the upper portion of the slide tube, or the transverse oil passages in the eccentric shaft and the connecting rod, resulting in lower workability and higher manufacturing cost.